1. Field of the Invention
The present invention generally relates to an improved drilling head which provides for the formation of a continuous seal about a drilling string, where said string consists of varying diameter drill string components. More particularly, the present invention is directed to a drilling head provided with stationary and rotatable sealing elements, which elements provide for continuous seal integrity about both drill pipe and collars while allowing ease of removal.
2. Background
Oil, gas, water and geothermal wells are drilled with a drill bit attached to a hollow drill string which passes down through a well casing installed in the well bore. A drilling head is usually attached to the top of the well casing or other blowout control equipment where it emerges from the ground to seal the interior of the well casing from the surface and thereby permit the forced circulation of drilling fluid or gas during drilling operations. In the more commonly used forward circulation drilling mode, the drilling fluid or gas is pumped down through the interior of the hollow drill string, out the bottom thereof, and upward through the annulus between the exterior of the drill string and the interior of the well casing and then out the drilling string side outlet. In reverse circulation, the drilling fluid, gas or air is pumped down the annulus between the drill string and the well casing and upward through the drill string.
Prior art drilling heads often have included a stationary body which carries a rotatable spindle that is rotated by a kelly driving the rotary drilling operation. A seal or packing, often referred to as a stripper packer or stripper rubber, is carried by the spindle to seal the periphery of the kelly or the sections of the drill pipe, whichever is passing through the spindle, and thereby confine the fluid pressure in the well casing and prevent the drilling fluid, whether liquid or gas, from escaping between the rotary spindle and the drill string.
Numerous stripper packers have been developed to provide rotational and slideable sealing of the drill string within the drilling head. The rotation of the kelly drill string, the frequent upward and downward movement of the kelly and the drill string during addition of drill pipe sections, and the pressures to which the drilling head is subjected, demand that the packing components of the drilling head be able to withstand continuous use without sacrificing seal integrity about the drill string.
As modern wells achieve ever greater depths, greater temperature and pressure are encountered, promoting the presence of high temperature, high pressure steams or water vapor at the drilling head. Such rigorous drilling conditions pose increased risks to rig personnel from accidental scalding, burns, or contamination with well fluids.
A variety of drill heads have been developed to address these problems of increasing temperatures and pressures. One such drilling head incorporates a rotatable stripper rubber which is placed above a stationary stripper rubber, where the upper, rotatable stripper rubber has a smaller inside diameter than the stationary rubber. In such a design, a seal is maintained against the incursion of steam or high temperature water when tripping the drill string out of the well, since a seal is constantly maintained for both the drill pipe and the drill collars.
Such prior art drilling heads, however, had a number of disadvantages. One such disadvantage is the cumbersome and time-consuming operation which was required to change the sealing components, e.g. the stripper rubbers. In prior art designs, the larger stationary rubber required a time intensive operation to unbolt or rebolt the rubber in the drilling head bowl. Further, such operation posed considerable hazard to rig personnel in the event steam or hot water bypassed a closed valve below the bowl. Further, such unbolting operations often resulted in the loss of a wrench or bolts into the well, thus requiring expensive removal.
Removal and replacement of the unbolted stripper rubbers is also tedious in prior art heads. Removal often required the use of a cat line and removal tool to unseat the rubber. Replacement of the rubber involved a similar time-consuming operation.